Mileage drop '92 S10 Blazer 4.3 auto

Gentlemen, I could use your assistance with this one. When I first got this blazer last March my fuel mileage was about 19 mpg. This past winter when the air temp went down to 0 degrees it
started getting hard to start, I have to hold the gas pedal to the floor, or it just cranks. And, my gas mileage has dropped to just over 15 mpg. I took it to the local shop and spent a ton of money, about $300 on a tune up and oil change. The money isn't the issue, according to the shop the spark plugs on the left, (drivers) side of the engine were showing that they were running rich while the plugs on the other side were ok. The codes in memory were 42 est problem, 43 esl and or sensor problem, 45 rich exhaust. When I got the truck back the shop told me that they thought the fuel pressure regulator was going bad, allowing fuel to run down the inside of the intake manifold, causing the hard starts. Apart from applying the old wash it, wax it, and burn it solution where do I go from here? The shop cleared the codes but did nothing to find the causes other than the tune up. I would hate to waste the money already spent by having the work repeated and still have the underlying problems. Any thoughts, input or, solutions would be a big help.
Lost in the wilderness,

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